Human skin, whatever its depth, falls within a narrow band of hues between red and yellow. Lighter and deeper skin differ mainly in brightness and saturation, not in hue. On a colour wheel, the direct opposite of that band is a blue-green: teal. Placing a colour beside its complement gives the strongest possible contrast in hue, so a warm face against a teal background separates even when both are equally bright.
There is also a practical reason. Daylight shadows are lit by blue sky, and artificial interior light is warm, so many real scenes already contain the two halves of the palette. The grade simply exaggerates and tidies what the camera recorded.
Where it came from
Grading a whole feature digitally became practical around 2000; the Coen brothers’ O Brother, Where Art Thou? is usually cited as the first Hollywood film finished that way. Once colourists could shift hues selectively across an entire film, the warm-against-cool split spread through action cinema, then into trailers, music videos and eventually phone presets. By the 2010s it was common enough to become a running joke among critics, which is why today’s versions tend to be more subtle.